Rotary piston and cylinder devices can take the form of an internal combustion engine, or a compressor such as a supercharger or fluid pump, or as an expander such as a steam engine or turbine replacement, and also a positive displacement device.
A rotary piston and cylinder device comprises a rotor and a stator, the stator at least partially defining an annular chamber/cylinder space, the rotor may be in the form of a ring, and the rotor comprising at least one piston which extends from the rotor into the annular cylinder space, in use the at least one piston is moved circumferentially through the annular cylinder space on rotation of the rotor relative to the stator, the rotor being sealed relative to the stator, and the device further comprising cylinder space shutter means which is capable of being moved relative to the stator to a closed position in which the shutter means partitions the annular cylinder space, and to an open position in which the shutter means permits passage of the at least one piston, the cylinder space shutter means comprising a shutter disc.
It is common practice to manage the clearances between moving components during operation by applying a relatively soft, friable or abradable coating to one of them, that is worn away by the other component which is relatively harder. Such coatings can for example be homogeneous coatings such as softer metals, or porous thermally-sprayed aluminium-based coatings. When worn away, these coatings are designed to break up into small particles to avoid damaging either component and minimise the clearance between them to reduce fluid leakage. Such coatings are typically used in jet engines and gas turbines to reduce leakage between the tips of rotating blades and the stationary shroud. In such an example scenario a largely continuous stationary surface (the shroud) is sealing against the relatively small surface of the radially outward tip of the rotating blade. A small clearance is desired to prevent gas leakage across the tip of the blade from the high pressure side of the blade to the low pressure side of the blade.
Rotary piston and cylinder devices can include some such areas, however a typical embodiment will also include areas of largely continuous close-running faces between the rotor and the stator. Such faces can be defined as those extending for at least 90° continuously over a width that is at least 1% of the total circumference. In cases of non-constant diameter the smallest diameter of a close-running edge should be used as reference, and in cases of curved faces the width should be the length of the curve defining a cross-section of the surface. In these areas the faces are rotating with respect to each other in a range of possible orientations.
Close running surfaces as described above are present in a number of locations in a rotary piston and cylinder device, as shown by the greyed out regions shown in FIGS. 2 and 3 on a possible compressor embodiment. It will be seen that some of the surfaces have cut-outs for ports or other requirements. It will be understood that other embodiments of a rotary piston and cylinder device are possible, and that the locations of close running surface pairs therein will vary, while the surface treatments disclosed in this patent will still apply.
Applying an abradable coating to one of those faces in an attempt to reduce gas leakage through the interface is apparent to one skilled in the art and familiar with the use of abradable coatings for common applications such as gas turbines. The relatively soft abradable will allow a very tight initial assembly, and will be eroded during operation to account for any thermal expansion, distortion or movement. Although either of the two mating surfaces can have the abradable coating applied, common practice is to apply abradable to the stationary surface to reduce imbalance following running-in of the device. This approach does not produce the desired results in the present scenario of close-running largely continuous faces, however, instead resulting in deep circumferential gouging of the abradable surface. This is likely to be caused by abraded debris not being able to escape from the close-running region, as the two surfaces are largely continuous, deepening grooves on each rotation. The remedial solution apparent to one skilled in the art is to add channels to the abradable coating to reduce build-up of debris. Such channels can take the shape of circumferential or axial, or largely helical grooves, the latter two extending across the face to allow worn material to escape from the close-running area to reduce gouging. The former option creates an interface similar to a labyrinth seal, as well as providing areas for particles to break up without damaging the close-running areas of the face.
This method can provide a suitable solution for some applications, but we have realised is not suitable for rotary piston and cylinder devices for the reasons described below. Circumferential grooves have little effect on axial fluid flow through the interface, but increase the rate of circumferential fluid flow (as fluid can flow through the grooves). Similarly the axial grooves have little effect on circumferential flow, but increases axial fluid flow through the interface. Helical groves increase both axial and circumferential fluid flow through the interface, but can offer more effective removal for particles of abraded coating.
We have devised an improved abradable surface for close-running surfaces in rotary piston and cylinder devices.